Machine Type Communication (MTC) is currently an important research topic in the 5th Generation (5G) mobile communication technology and an important application area in future wireless communications. In Machine-to-Machine (M2M) topic, a research sub-topic of Narrow Band-Internet of Things (NB-IoT) has been proposed for characteristics of terminals such as low cost, low power consumption, low mobility and low throughput. That is, low-throughput wireless communication services can be provided for low-cost NB-IoT terminals (or User Equipment (UE)) within a 200 KHz frequency band.
In order to reduce signaling overhead and power consumption at NB-IoT terminals, two optimization schemes for Cellular Internet of Things (CIoT), have been introduced in the current researches.
1. Control Plane (CP) Optimization Scheme: No Data Radio Bearer (DRB) and no access stratum (AS) security context will be established between a terminal and a network. Data is encapsulated into non-access stratum (NAS) Protocol Data Unit (PDU) and attached to CP signaling for transmission. This scheme can significantly save signaling but has a limited data packet length for each transmission. A large data packet has to be partitioned into a number of small packets for transmission. In case of a poor coverage, such packets can be lost, such that a receiver cannot receive a complete data packet.
2. User Plane (UP) Optimization Scheme: A complete connection is established between a terminal and a network and data is transmitted using DRBs. After completion of data transmission, a bearer information context and an AS security context can be maintained at the terminal and the network side in a suspending process. For a subsequent data transmission, the terminal and the network resume the maintained contexts in a resumption process to continue transmitting data using the DRB. This scheme is substantially the same as the existing process and has no significant limitation on the data packet length. However, it has a limited effect in saving signaling.
Further, a process for negotiating capabilities between a terminal and a network has been introduced by the current research. Preferred Network Behavior indication information can be carried in an Attach Request message, indicating operation schemes the terminal can support and would recommend. A network element Mobility Management Entity (MME) at the network side will include Supported Network Behavior in an Attach Accept message for indicating operation schemes the network can support.
When the UP scheme is adopted for the terminal-network negotiation, when the terminal has uplink data to send, it needs to trigger a resumption process for air interface and network-side bearers. When the network side has downlink data to send, it will page the terminal first and then trigger the terminal to initiate a resumption process for air interface and network-side bearers.
In the up-to-date standard, for the connection resumption process in the UP optimization scheme, a Non-Access Stratum (NAS) of a terminal, in an idle state with a suspension flag, sends an instruction to resume a connection to an Access Stratum (AS) of the terminal and buffers a NAS signaling for service request. The AS of the terminal will attempt to transmit a connection resumption request message to an AS of a base station. In this scenario, there can be several possible operations and interactions in the NASs and ASs of the base station and the terminal, as follows.
1. Normally, upon receiving the connection resumption request message, the AS of the base station will transmit a connection resumption message to the terminal, indicating that the connection has been successfully resumed for the terminal. The AS of the terminal then sends an indication to the NAS of the terminal, indicating that the connection has been successfully resumed (i.e., “RRC connection has been resumed”). The NAS of the terminal will discard the buffered NAS signaling for service request and terminate the NAS process.
2. There can also be the following scenarios which can be considered as abnormal.
a) In some cases, the AS of the terminal can not transmit the connection resumption request message for failing to pass an Access Baring test. In this case, the AS of the terminal sends an indication to the NAS of the terminal, indicating a failure to resume the connection (i.e., “failure to resume”). The NAS of the terminal can reuse or refer to the processes for connection establishment failure (i.e., “failure to establish”) and congestion control.
b) In some other cases, the AS of the terminal can not transmit the connection resumption request message because the current serving base station does not support the UP optimization scheme. In this case, the AS of the terminal will clear the stored AS context and send an indication to the NAS of the terminal, indicating a failure to resume the connection (i.e., “RRC connection resumption has failed”). The NAS of the terminal will send the buffered NAS signaling for service request to the AS, which then performs a normal connection establishment process.
c) In some other cases, if some abnormality occurs after the AS of the base station has received the connection resumption request message, e.g., when the AS of the base station fails to find a context associated with a terminal resumption ID, the AS of the base station can also transmit a connection establishment message to the terminal, instructing the terminal to clear the buffered contexts and perform a normal connection establishment. In this case, the AS of the terminal will send an indication to the NAS of the terminal, indicating a failure to resume the connection (i.e., “RRC connection resumption has failed”). The NAS of the terminal will send the buffered NAS signaling for service request to the AS, which then performs subsequent processes and transmits the NAS signaling to the base station for establishing a S1 interface later.
d) In some other cases, after receiving the connection resumption request message, the AS of the base station can transmit a connection rejection message to the terminal, e.g., due to network congestion. Alternatively, the terminal can not receive any response to the connection resumption request message due to an abnormity in a radio link, which causes timeout of timer T300. In either case, the AS of the terminal sends an indication to the NAS of the terminal, indicating a failure to resume the connection (i.e., “failure to resume”). The NAS of the terminal can reuse or refer to the processes for connection establishment failure (i.e., “failure to establish”) and congestion control. Additionally, the connection rejection message transmitted from the AS of the base station to the terminal can optionally include suspension information. If the suspension information is included, the AS of the terminal will maintain the stored AS context information; otherwise the AS of the terminal will clear the stored AS context.
There can be the following problems associated with the above processes and interactions.
1. In the current standard, when the NAS of the terminal receives a “failure to resume” indication, the NAS of the terminal can reuse or refer to the processes for connection establishment failure and congestion control. That is, the NAS of the terminal does not distinguish between “failure to establish” and “failure to resume”. When the terminal makes a connection resumption request, the NAS of the terminal is in an idle state with a suspension flag. If the terminal receives a “failure to resume” indication at this time, it needs to consider at least how to handle the stored NAS context (including the suspension flag). On one hand, the NAS of the terminal can clear the suspension flag and return to a normal idle state. On the other hand, the NAS of the terminal can alternatively stop the current process and maintain the current state.
2. Further, in the case where the AS of the terminal receives a connection rejection message, it is different from the existing rule that an extended wait timer is applied only to terminals supporting delay-tolerable access. If the current terminal is a NB-IoT terminal, its AS always sends the extended wait timer to its NAS, such that it can be applied at the NAS. However, conventionally, the NAS will determine whether the terminal is currently initiating a low-priority access when handling the timer. If so, the NAS will start the timer and wait for a period of time (i.e., starting a timer T3346 with the “extended wait time” value); otherwise it will discard the timer. That is, currently the use of the extended wait timer by the NAS of the terminal is conditional. If the condition is not met, the requirement that the AS shall apply extended wait timers to all connection rejection scenarios for NB-IoT terminals can not be satisfied.
3. Further, in a special case where the AS of the terminal receives a connection rejection message that carries the suspension information, the AS of the terminal will maintain the stored context information. If at this time the NAS of the terminal receives a “failure to resume” indication from the AS, clears the suspension flag and returns to the normal idle state, there will be inconsistency between the AS and NAS of the terminal. On the other hand, in a special case where the AS of the terminal receives a connection rejection message that carries no suspension information, the AS of the terminal will clear the stored NAS context. If at this time the NAS of the terminal receives a “failure to resume” indication from the AS, stops the current process and maintains the current state, there will be another type of inconsistency between the AS and NAS of the terminal. When the NAS of the terminal sends an instruction to resume the connection to the AS of the terminal subsequently, the AS cannot proceed with subsequent process as it does not have any AS context stored therein.
4. In addition to the scenarios described above, the standard does not very clearly specify the AS operations and the AS-NAS interactions when other abnormalities occurs at the AS of the terminal during the connection resumption process, such as reestablishment failure due to Radio Link Failure (RLF) or timeout of timers other than T300.
The problematic AS-NAS interactions as described above can lead to problems such as inconsistent operations between the AS and the NAS.
For the connection resumption process in the UP optimization scheme, before the terminal triggers the resumption process, it is assumed that various context information required for the UP optimization have already been stored in the terminal, base station and related core network elements. It is important problem to be solved regarding how to identify the context information such that the terminal and the network side can use consistent identifications to search for and resume the correct context information. In the up-to-date standard, a 40-bit full-text resume identity has been defined. This full-text resume identity is allocated by the base station to the terminal during the suspending process. In accordance with the instruction from the network side, the terminal can carry the 40-bit full-text resume identity or a 24-bit truncated resume identity in the connection resumption request message and transmit it to the base station.
However, the above scheme for transmitting the resume identity has the following problems.
1. In the up-to-date standard, the base station shall construct the 40-bit full-text resume identity in a format containing 20-bit base station information and 20-bit terminal/user information. In a simplest way, the base station can place the 20-bit terminal/user information in the 20 Most Significant Bits (MSBs) of the full-text resume identity and the 20-bit base station information in the 20 Least Significant Bits (LSBs), or vice versa. However, if the base station adopts another, flexible construction scheme, the terminal would not be able to know information on the specific format of the 40-bit full-text resume identity. For example, the terminal would not be able to know whether the 20-bit terminal/user information is placed in the MSBs, LSBs or somewhere in the middle of the 40-bit full-text resume identity.
2. On the other hand, in accordance with the instruction from the network-side, the terminal can determine to transmit a 24-bit truncated resume identity to the base station. In a simplest way, the base station can extract the 24 MSBs from the 40-bit full-text resume identity, meaning that the 20-bit terminal/user information and 4 bits of the base station information are used to construct the truncated resume identity. However, if the terminal requires a more flexible scheme, e.g., requiring more base station information to be carried in the resume identity in e.g., a mobility scenario, the terminal cannot notify the base station of the specific format of the 24-bit resume identity, e.g., whether to include 20 bits or less of the base station information, whether to include 20 bits or less of the terminal/user information, or whether to include other information. In other words, the standard cannot provide such flexibility in transmission of the resume identity.
Hence, the transmission of the resume identity can lead to problems such as inconsistent interpretation or lack of flexibility.
There is currently no effective solution in the related art to the problems associated with inconsistent operations in the UP optimization.